A newborn boy is found to have a serum TSH above and serum thyroxine below the normal range for his age. The patient was born at 39 weeks gestation via spontaneous vaginal delivery. His mother has no medical conditions. On examination, the patient's length and weight are normal, and he has no apparent congenital abnormalities. Ultrasound of the neck shows a normal-sized thyroid gland in the appropriate location. Which of the following is the most likely cause of this patient's condition?

Question

A newborn boy is found to have a serum TSH above and serum thyroxine below the normal range for his age.  The patient was born at 39 weeks gestation via spontaneous vaginal delivery.  His mother has no medical conditions.  On examination, the patient's length and weight are normal, and he has no apparent congenital abnormalities.  Ultrasound of the neck shows a normal-sized thyroid gland in the appropriate location.  Which of the following is the most likely cause of this patient's condition?

Quizplus · Accepted Answer

11ec160b_3cb4_b655_9249_f10a5a27802e_MD0001_00 This patient has an elevated serum TSH and decreased thyroxine (T4) level, findings consistent with primary hypothyroidism. The most common congenital causes are thyroid dysgenesis (eg, ectopy, hypoplasia) and TSH resistance. Less common, transient etiologies are usually due to maternal conditions (eg, iodine excess, antithyroid medications). Given that this patient's thyroid is normal in size and location and the mother has no medical conditions, the most likely cause is thyroid gland resistance to TSH. TSH is normally produced by the anterior pituitary and binds to TSH receptors on follicular thyroid cells to stimulate T4 production. In TSH resistance, a mutation in the TSH receptor gene leads to thyroid insensitivity to TSH and reduced synthesis of T4. Low T4 leads to positive feedback to the pituitary, further increasing TSH levels. Newborns are usually asymptomatic due to placentally transferred maternal T4 (although thyroid hormones do not readily cross the placenta, enough T4 can usually cross to prevent development of overt symptoms); diagnosis is typically via newborn screening. Treatment is lifelong levothyroxine. (Choice A) Central hypothyroidism (eg, panhypopituitarism) causes low serum T4 due to decreased production of TSH by the anterior pituitary. This patient's elevated TSH is inconsistent with central hypothyroidism. (Choice B) Generalized resistance to thyroid hormones due to a thyroid hormone-receptor mutation may lead to hypothyroidism due to insensitivity to T4. However, this condition causes elevated T4 and TSH. (Choice D) Thyroid hormone-binding globulin deficiency is a benign, euthyroid state characterized by a low total T4 but normal free T4 and TSH levels. (Choice E) Transplacental transfer of TSH receptor-stimulating antibodies in a mother with Graves disease can cause transient neonatal hyperthyroidism (neonatal Graves disease). Neonatal symptoms include tachycardia and irritability, and laboratory evaluation would show high T4 and low TSH. Educational objective: TSH resistance due to a mutation in the TSH receptor gene presents with congenital hypothyroidism, which is characterized by increased TSH and low thyroxine. The thyroid gland is normal in size and location.

A Newborn Boy Is Found to Have a Serum TSH